Because of a lattice mismatch between a germanium lattice and a silicon lattice, epitaxial growth of germanium directly on a silicon wafer results in a high density of dislocations at the Ge/Si interface. These dislocations are detrimental to the electrical performance of any germanium device fabricated on a silicon substrate. In order to reduce the dislocations at the Ge/Si interface, a germanium epitaxial lateral overgrowth (ELO) technique has been used.
Li et al., demonstrated that germanium may be selectively grown on Si(100) through openings in a SiO2 nanotemplate by solid source MBE. Germanium islands grow in the template windows and coalesce on top of the template, forming an epitaxial ELO layer. Cross sectional TEM images show that germanium seeds and the ELO layer are free of threading dislocations. Only stacking faults are generated, but terminate within 70 nm of the Ge-Si interface, while twins along {111} planes are observed in the ELO layer. Li et al., Selective growth of Ge on Si(100) through vias of SiO2 nanotemplate using solid source molecular beam epitaxy, Applied Physics Letters, Vol. 83, No. 24, 5032-5034, (2003), and Li et al., Heteroepitaxy of high-quality Ge on Si by nanoscale Ge seeds grown through a thin layer of SiO2, Applied Physics Letters, Vol. 85, No. 11, 1928-1930, (2003).
Langdo et al., High quality Ge on Si by epitaxial necking, Applied Physics Letters, Vol. 76, No. 25, 3700-3702, (2000), demonstrated that germanium grown selectively on a SiO2/Si substrate in 100 nm holes by chemical vapor deposition is nearly perfect at the top surface, compared to conventional germanium lattice-mismatched growth on planar silicon substrates. The threading dislocations generated at the Ge/Si interface are blocked at the oxide sidewall by the epitaxial necking mechanism. Defects at the germanium film surface only arise at the merging of epitaxial lateral overgrowth fronts from neighboring holes.
Although the two techniques use different growth methods, i.e., MBE and CVD, both produce a dislocation-free epitaxial lateral overgrowth layer. However, the twin planes are generated on the ELO layer because of the merging of ELO fronts from neighboring holes. Electrical characteristics of such germanium devices fabricated on the ELO germanium layer are not any better than devices fabricated on germanium layers directly grown on silicon wafers. This is because the twin plane is also a crystal defect that, electrically, behaves similarly to a dislocation. FIG. 1 depicts a cross-section diagram 10 of a prior art germanium layer 12 grown on silicon 14, through windows 16 in a SiO2 layer 18, demonstrating the principles of epitaxial necking showing zero threading dislocations at the germanium film surface. However, twin planes 20 are generated as a result of merging of epitaxial lateral overgrowth fronts from neighboring windows.
In our related application, identified above, we presented a germanium photo detector structure that is twin plane free and has device electrical performance unaffected by dislocations. The basic concept for our prior process, depicted generally at 22 in FIG. 2, includes fabrication of a silicon CMOS device on a silicon substrate wafer 24 prior to germanium device fabrication. CMOS fabrication includes N+ ion implantation to form an N+ layer 26 for the bottom electrode of the photodiodes. A layer of SiO228 is deposited and CMP planarized. Contact holes are formed in the SiO2 layer. After contact hole 30 formation, selective in situ N+-doped epitaxial germanium 34 is grown in the contact holes. The threading dislocations 36 rise up on (111) planes in <110>directions, and make a 45° angle to the underlying Si(100) substrate. Consequently, the dislocations are effectively contained inside the holes. Continued germanium deposition results in formation of intrinsic germanium layer 38 by epitaxial lateral overgrowth (ELO), and is followed by ELO growth of a P+ germanium layer 40. A layer 42 of polysilicon or In2O3—SnO3, (ITO) is deposited. The final prior art structure is shown in FIG. 3. Because the germanium ELO exhibits many facets, as shown in the SEM image of FIG. 4, formation of an in situ boron doped germanium layer is preferred in this step.
This previous disclosure offers a simple method to fabricate germanium photodetectors with defect-free germanium film. However, in cases where interface leakage is prevalent, the performance of the germanium photodetector is likely limited because of the abrupt interface between germanium and oxide.